The present invention relates to a composition and method for controlling plant diseases caused by fungi, in particular Botrytis cinerea and Alternaria alternata, which utilize substances derived from natural sources.
Gray mold caused by Botrytis cinerea and black mold caused by Alternaria alternata are serious problems in many crops around the world, both before and after harvesting. These pathogens attack many crops including fruits, vegetables and ornamental plants. Crops are attacked whether grown in the open or under cover. If not adequately controlled, such gray mold and black mold can cause substantial pre-harvest losses, as well as postharvest losses during transport and storage. The pathogens attack all parts of the plants including flowers, fruits, leaves, stems, branches, bulbs or seeds.
The use of chemical fungicides in an attempt to prevent or reduce losses due to such pathogens is fairly widespread, although they give rise to ecological and consumer concerns. Moreover, B. cinerea has developed resistance to common fungicides e.g. to dicarboximides and benzimidazoles. In many places, farmers do not have any solution for the problem because of such resistance.
Chitin, which occurs in fungi, yeasts, marine invertebrates and arthropods, may be regarded as cellulose in which the C.sup.2 --OH groups have been conceptually replaced by acetamido groups. Chitosan is deacetylated chitin. .beta.-glucan is a generally linear polymer composed essentially of .beta.(1-3) D-glucan, but may also be branched and include side-chains of .beta.(1-6) D-glucan. Soluble oligosaccharides are obtained by partial degradation of these polymers. It has moreover been demonstrated that oligosaccharides including those derived from glucan, chitosan and chitin, and which are released from fungal cell walls during plant-fungus interactions are elicitors of plant defense mechanisms (M. G. Hahn et al. in Mechanisms of Plant Defence Responses, B. Fritig and M. Legrand (eds.), Kluwer Academic Publishers (Netherlands), 1993, pages 99-116), while these and other oligosaccharides also have effects on growth and development that are not obviously related to disease resistance (S. C. Fry et al, Plant Physiol 103: 1-5 (1993)). Thus, it is evident that the literature on such oligosaccharides provides only limited information as to whether any particular oligosaccharides will be effective in fighting plant diseases. Further, there is no evidence in the literature that either chemical agents generally, or oligosaccharides in particular, which are effective in controlling certain plant disease will control others; e.g. S. Hirano and N. Nagao in Agric. Biol. Chem. 53, 3065-3066 (1989), reported inter alia that chitosan oligosaccharides inhibit the radial growth in vitro of Alternaria alternata (except certain pathotypes) but not Botrytis cinerea; and also reported that chitosan (1.0 mg/ml) inhibited the radial growth in vitro of many pathogenic fungi including Botrytis cinerea and Alternaria alternata.
In vivo, chitosan was shown to inhibit plant disease in low concentrations only in the case of Fusarium solani in pea plants in less than 10 .mu.g/ml (L. A. Hadwiger and J. M. Beckman, Plant Physiol. 66, 205-211 (1980)), and Pythium aphanidermatum in cucumber plants in 100 .mu.g/ml (A. El Ghaouth et al, Phytopathology, 84, 313-320 (1994)). In Botrytis cinerea and in Rizopus stolonifer, chitosan was shown to inhibit in high concentrations as much as 10 mg/ml (A. El Ghaouth et al, Phytopathology, 82, 398-402 (1992). Allan, C. R., et al., Experimental Mycology 3:285-287 (1979), and Stossel, P., et al., Phytopath. Z., 111:82-90 (1984), both report on antifungal activity of chitin and chitosan, but not of hydrolyzed chitin or chitosan. The mechanism by which chitosan inhibits fungal growth has not been fully elucidated. Kendra, D. F. et al., Experimental Mycology 8: 276-281 (1984), report the results of studies on the anti-Fusarium solani activity of chitosan oligomers.
JP 62-198604 describes chitosan hydrolyzates, MW .ltoreq.3000, as a control agent for pear black spot due to in particular Alternaria Alternata Japanese pear pathotype. Neither this published document nor any other, to the best of the inventors' knowledge, describes chitosan hydrolyzates as a control agent for diseases caused by Botrytis cinerea.
U.S. Pat. No. 5,374,627 describes a method for treating plant diseases with a composition including a chitosan hydrolyzate and acetic acid, but does not disclose that such hydrolyzate has any plant disease controlling activity in absence of acetic acid. U.S. Pat. No. 4,970,150 describes and claims an enzymatic method for preparing low molecular weight chitosan oligosaccharides under specific conditions; the product is said to have antibacterial properties, no mention is made of antifungal properties.